In recent years, along with a tendency to high brightness and whitening of a light-emitting device such as LED, a light-emitting device employing LED has been used for a backlight of a mobile phone, a large sized liquid crystal TV, etc. In order to apply a LED lamp to various applications, it becomes important to obtain a white emission. As typical systems to realize a white emission by a LED lamp, four systems may be mentioned i.e. a system of using three LED chips which emit blue, green and red colors, respectively, a system of combining a blue-emitting LED chip with a yellow or orange-emitting phosphor, a system of combining a blue-emitting LED with a phosphor exciting red and green by the emission, and a system of combining an ultraviolet-emitting LED chip with a blue, green and red-emitting three-color mixed phosphor.
As the above-mentioned system of combining a phosphor, a cannonball structure is known which is prepared by casting a clear resin such as an epoxy resin or silicone resin having a phosphor mixed thereto, followed by solidification to form a resin layer containing the phosphor. Further, a structure is known which is prepared by mounting a LED chip on a substrate having a circuit pattern formed on its main surface and further forming on this substrate a sealing portion made of a clear resin. In such a LED lamp, a light reflection layer made of e.g. silver is formed on the substrate around the mounted LED chip. And, by such a light reflection layer, an emission from the LED chip radiated to the substrate side, or a fluorescence emitted under excitation from the phosphor, is reflected to the forward direction thereby to improve the light extraction efficiency.
However, silver is likely to be corroded, and if it is left to stand, a compound such as AgS is likely to be formed, whereby the light reflectance tends to deteriorate. Therefore, it has been attempted to prevent such a deterioration of the reflectance by forming a resin sealing layer on the silver, but with an epoxy resin or silicone resin which is usually used as the resin sealing agent, the sealing property is weak, and it has been difficult to employ it for products which are required to have a reliability for a long period of time.
Accordingly, in order to prevent corrosion of a silver conductor layer, a method has been proposed to coat the surface of silver with a resin such as a silicone resin, an acryl resin, an epoxy resin or an urethane resin (Patent Document 1). However, even by such a method, moisture or a corrosive gas is likely to enter into the resin or from the interface between the silver conductor layer and the resin, and as the time passes, the silver conductor layer is corroded, and thus such a method has not been useful for products which are required to have a reliability for a long period of time.
On the other hand, as a method to increase the reflectance without using a silver conductor, it has been proposed to use an alumina material or the like having a high reflectance, but firing at a high temperature exceeding 1,000° C. is required, and thus there has been a problem that the load on the production process is substantial.